UDK 620.193:624.94
Original scientific article/Izvirni znanstveni ~lanek
ISSN 1580-2949
MTAEC9, 48(1)51(2014)
A. ^ESEN et al.: CORROSION PROPERTIES OF DIFFERENT FORMS OF CARBON STEEL ...
CORROSION PROPERTIES OF DIFFERENT FORMS OF
CARBON STEEL IN SIMULATED CONCRETE PORE
WATER
KOROZIJSKE LASTNOSTI RAZLI^NIH OBLIK JEKEL V
SIMULIRANI PORNI VODI BETONA
Ale{ ^esen1, Tadeja Kosec1, Andra` Legat1, Violeta Bokan-Bosiljkov2
1Slovenian
2Faculty
National Building and Civil Engineering Institute, Dimi~eva 12, 1000 Ljubljana, Slovenia
of Civil and Geodetic Engineering, University of Ljubljana, Jamova cesta 2, 1000 Ljubljana, Slovenia
[email protected]
Prejem rokopisa – received: 2013-03-13; sprejem za objavo – accepted for publication: 2013-04-23
Carbon steel, such as concrete-reinforcing steel, tends to undergo corrosion processes when exposed to certain environmental
actions. These are the carbonation of concrete and the ingress of chlorides into the concrete from the environment. Many times,
the carbonation and chloride contamination are simultaneous processes leading to a harsh corrosion environment and
subsequent corrosion problems. Monitoring the state of corrosion is thereby a very useful and powerful tool for following and
evaluating the lifetime of reinforced concrete structures. Electrochemical measurements were performed to investigate different
forms of carbon steel in simulated concrete pore water at different pH values with and without the presence of chlorides.
Morphological characteristics of three different types of carbon steel were studied and SEM/EDX and Raman analyses of the
corrosion products were performed. It was found that steel in the form of a sheet has a higher corrosion resistivity than a steel
wire and a steel rod, and that the steel rod has a higher corrosion resistivity than the steel wire. The corrosion layer on carbon
steel is very diverse; several morphologies were found and analyzed.
Keywords: carbon steel, metallography, corrosion, concrete pore water
Malooglji~no jeklo, kot je jeklena betonska armatura, je izpostavljeno korozijskim procesom zaradi vplivov okolja. To sta
karbonatizacija betona ter vdor kloridnih ionov iz okolja. Mnogokrat se karbonatizacija in vdor kloridov zgodi isto~asno, kar
privede do hitrega korozijskega propadanja. Spremljanje korozije na objektih je zato zelo dobro orodje za napoved in oceno
preostanka trajnostne dobe nekega objekta. Elektrokemijske preiskave smo izvedli v simulirani raztopini porne vode v betonu
pri razli~nih pH vrednostih ter brez in v prisotnosti kloridnih ionov. Dolo~ili smo mikrostrukturne zna~ilnosti posameznih vrst
oglji~nih jekel ter morfologijo korozijskih produktov z EDX/SEM-analizo ter Ramansko spektroskopijo. Ugotovili smo, da je
najbolj korozijsko odporno oglji~no jeklo v obliki folije, sledi palica, najslab{e korozijske lastnosti pa ima `ica iz oglji~nega
jekla. Korozijski produkti na jeklu so razli~nih morfolo{kih oblik in sestav.
Klju~ne besede: oglji~no jeklo, metalografija, korozija, porna voda betona
1 INTRODUCTION
The service life of a reinforced concrete structure
depends on the corrosion state of the reinforcing steel
that is embedded in the concrete.1 There are numerous
ways to prolong the service life of a structure, among
them also the measures related to the properties of structural materials such as using high-performance concretes
with improved properties2–4 or using a more durable
reinforcement material like the corrosion-resistant
steel.5–10 However, the use of the common carbon steel as
a concrete reinforcement is still the most frequent and
economical. Thus, reliable corrosion monitoring is
essential to assess the remaining life-time of a structure,
to help select an optimum rehabilitation process and
evaluate its efficiency.10–12 With a corrosion monitoring
system we aim to detect the changes in the reinforcement
condition, evaluate the corrosion rates and determine the
types and causes of corrosion.13–15
It is of great importance to know the corrosion stages
and mechanisms in order to evaluate the intensity of
corrosion. Thereby, two extreme conditions were chosen
Materiali in tehnologije / Materials and technology 48 (2014) 1, 51–57
to be studied in the present research. These are the
ingress of chloride ions and the carbonation of the
cement matrix. At a high alkalinity and high pH, the
passive film on the carbon steel protects the metal from
corrosion.13 The oxide layer consists of firm and
adherent Fe2O3.13 When the same steel is subjected to a
low pH, or a pore solution containing chlorides, the
passivity is lost. Anodic reactions of carbon steel are
dissolving iron through many possible reactions:
Fe ® Fe2+ + 2 e–
(1)
2+
Fe + 2 OH– ® Fe(OH)2
(2)
4 Fe(OH)2 + O2 ® 4 g – FeOOH + 2 H2O
(3)
2+
–
Fe + 2 Cl ® FeCl2
(4)
FeCl2 + H2O + OH– ® Fe(OH)2 + 2 Cl– + H+ (5)
4 Fe(OH)2 + 2 H2O + O2 ® 4 Fe(OH)3
(6)
2 Fe(OH)3 ® Fe2O3 · H2O + 2 H2O
(7)
The cathodic reaction in such cases can be a reduction of the oxygen present in the electrolyte:
O2 + 2 H2O + 4 e– ® 4 OH–
(8)
On the other hand, carbon dioxide can reduce the
protective role of concrete due to a reaction with the
51
A. ^ESEN et al.: CORROSION PROPERTIES OF DIFFERENT FORMS OF CARBON STEEL ...
hydrated cement paste, leading to a pH decrease and a
subsequent loss of passivity and to a corrosion initiation.13,16 The corrosion reaction in such an environment
is accelerated.
The aim of the present study is to compare the
corrosion properties of different forms of carbon steel,
differing by their microstructural properties. Their
corrosion properties in a simulated concrete-pore-water
solution are also evaluated with the presence of chloride
ions. Different spectroscopic techniques are used to
study morphological and mineralogical characteristics of
the corrosion products on steel.
2 EXPERIMENTAL WORK
2.1 Materials and surface preparation
Three types of samples were chosen for the study,
namely:
1) Carbon-steel sheets 240 μm thick with the sections of
different dimensions. The wire for the electrical contact on the carbon-steel-sheet specimen was attached
at the side of the sheet plates beforehand. It was
protected by epoxy paint.
2) Carbon-steel rods with a diameter of 5.0 mm. The
wire for the electrical contact was attached at the top
of the rod prior to the measurements.
3) Carbon-steel wires with a diameter of 0.8 mm. The
cross-section of the wire was exposed to the electrolyte.
Before each measurement, the wires and steel rods
were abraded with the 1200-grit emery paper, degreased
with acetone and then well dried.
2.2 Electrochemical measurements
The electrochemical measurements were performed
in different types of the solution:
1) the 0.01 M calcium hydroxide solution, simulating
the concrete pore water at pH 12.8
2) the 0.01 M sodium tetraborate solution, simulating
the pore water of the carbonated concrete and a
presence of chloride ions (pH 9.2, containing 0.58 %
of NaCl)
A laboratory-made three-electrode corrosion cell was
used with an approximate volume of 300 cm3. The
specimen was prepared in such a way that the exposed
surface presented the working electrode. For the electrochemical tests a potentiostat/galvanostat PGSTAT100,
the floating version, Metrohm, Netherlands, 2010,
expanded with the NOVA module was used.
After the initial stabilization 2 h at the open-circuit
potential (OCP), 3 subsequent linear polarization measurements at ±20 mV vs. OCP with a scan rate of 0.1 mV/s
were performed. Finally, the potentiodynamic curve was
measured in the range of –250 mV vs. OCP to 0.6 V at a
scan rate of 1 mV/s.
52
2.3 SEM/EDX analysis
For the SEM/EDX analysis, a sample of the
carbon-steel sheet was embedded into the cement mortar.
The carbon-steel sheet was exposed to wetting and
drying cycles during a 12-week exposure. During the
first 6 weeks, the cycle period consisted of 2 days of
wetting with distilled water and 5 days of drying the
mortar specimen. During the second 6-week period, the
cycles consisted of wetting the specimen with a 3.5 %
NaCl solution. After the exposure, the mortar was
detached from the steel-sheet sensor and the corrosion
products were investigated using SEM and EDS. At the
end of the exposure, the carbon-steel sheet was detached
from the mortar cover, rinsed with distilled water and
dried. The surface morphology was inspected and
analyzed with a low-vacuum scanning electron
microscope (SEM, JSM 5500 LV, JOEL, Japan) at the
acceleration voltage of 20 kV. The microscope was
equipped with energy dispersive spectroscopy (Inca,
Oxford Instruments Analytical, UK). The EDS analysis
was performed at the acceleration voltage of 20 kV.
2.4 Raman analysis
The Raman spectra were obtained with a Horiba
Jobin Yvon LabRAM HR800 Raman spectrometer
coupled with an Olympus BXFM optical microscope.
The measurements were performed using a laser excitation line 633 nm, a 100-times objective lens and a
600-grooves per milimetre grating, which gave a spectral
resolution of 2 cm–1 per pixel. The power of the laser was
set at 0.14 mW. A multi-channel air-cooled CCD detector was used, with the integration times of between 20 s
and 30 s. The spectra presented are without any baseline
correction.
2.5 Metallographic examination
The samples were first grinded up to grades 2000,
then they were polished up to 4000 and finally a paste
0.5 μm was used. The etching for uncovering the
microstructure was performed in a 3 % mixture of HNO3
in ethanol for 20 s. The samples were then immediately
rinsed with ethanol and dried with air. A CARL ZEISS
AXIO Imager M2m optical metallographic microscope
was used to study the microstructure of the steel specimens. Metallographic specimens were prepared and
investigated in the longitudinal and transverse directions
of the castings. The results of the directions representing
the exposed surfaces in the electrochemical study were
obtained.
3 RESULTS AND DISCUSSION
In order to evaluate the corrosion properties of the
three different types of carbon steel, electrochemical
Materiali in tehnologije / Materials and technology 48 (2014) 1, 51–57
A. ^ESEN et al.: CORROSION PROPERTIES OF DIFFERENT FORMS OF CARBON STEEL ...
experiments were conducted in a simulated concretepore-water solution with pH 12.8.
The comparison of the electrochemical properties at
pH 12.8 and the ones in the simulated carbonated
environment at pH 9.2 and with the presence of 0.58 %
of chlorides is presented as well. For a further evaluation
of the corrosion behavior, the samples were characterized after a 12-week exposure to the cycling conditions.
After that, the surface of the exposed sensor from the
carbon-steel sheet was examined with EDX/SEM and the
Raman technique.
3.1 Metallographic examination
Metallographic images of the three shapes of the
steel specimens are presented in Figure 1, namely, a
longitudinal view of the steel sheet (a), the rod (b) and a
cross-section of the wire (c).
The steel sheet has a well-defined microstructure. It
consists of ferrite crystal grains that vary in size from 20
μm to 40 μm (Figure 1a). Small spheroid-shaped carbides are distributed at the edges and in the crystal grains.
It is assumed that the carbides might affect the corrosion
properties of the steel sheet.
The microstructure of the rod in the longitudinal
direction is mostly ferritic (Figure 1b). The amount of
perlite is very small due to a low carbon content. Crystal
grains are of the size of between 20 μm and 45 μm and
are extremely pure towards the surface of the normalized
steel rod. The content of the sulfide inclusions increases
towards the core of the rod.
The cross-section of the wire has a lamellar microstructure due to the cold-worked procedure (Figure 1c).
The microstructure consists of cementite and ferrite that
were induced out of the perlite microstructure. The size
of the cementite lamellas is estimated to be several nanometres.17 The corrosion performance of the wire is
expected to be more susceptible to corrosion in the
cross-section than in the longitudinal direction.
3.2 Electrochemical measurements
During the stabilization process, the open-circuit
potential was measured as a function of time. Figure 2
represents the open-circuit potential curves of the three
different types of the steel specimens, namely, the carbon
sheet, the carbon-steel rod and the carbon-steel wire,
immersed in a simulated concrete-pore-water solution
with pH 12.8. All the measured curves showed a similar
electrochemical behavior.
The corrosion potential, Ecorr, in all the cases moved
to more negative values. After two hours of the immersion it stabilized at –0.280 V for the carbon-steel sheet
and at –0.279 V for the carbon-steel wire. The corrosion
potential of the carbon-steel rod was the lowest and
Figure 1: Metallographic images of the three forms of carbon steel: a)
steel sheet – a longitudinal view, b) steel rod – a longitudinal view, c)
steel wire – a cross-section
Slika 1: Metalografski posnetki treh razli~nih oblik oglji~nega jekla:
a) vzdol`ni prerez jeklene plo{~e, b) vzdol`ni prerez jeklene palice in
c) prerez `ice
Materiali in tehnologije / Materials and technology 48 (2014) 1, 51–57
Figure 2: Open-circuit potential measurements for the three forms of
carbon steel: steel sheet, steel rod and steel wire, immersed in
simulated pore water, pH 12.5
Slika 2: Meritve pri potencialu odprtega kroga za jekleno plo{~o,
jekleno palico in prerez `ice, potopljene v simulirano raztopino
betona, pH 12,5
53
A. ^ESEN et al.: CORROSION PROPERTIES OF DIFFERENT FORMS OF CARBON STEEL ...
stabilized at –0.315 V after two hours of the immersion.
The observed decrease in the value of Ecorr in time might
be a result of the formation of an adsorbed layer at the
interface of the carbon steel/electrolyte in the simulated
concrete-pore-water solution. However, the Ecorr evolution is quite regular, indicating that a stable layer was
formed on the investigated steel surfaces. The corrosion
potentials, Ecorr, are relatively high, since the corrosion
potentials of corroding steels are reported to be as low as
–0.7 V.16
At a low pH and in a chloride-contaminated environment, the corrosion potentials moved to a more negative
direction (Figure 3). After a exposure 1 h, it was
–0.5605 V for the steel sheet, –0.620 V for the wire and
–0.624 V for the rod. The lower values for all the
investigated samples, compared to the values at pH 12.8,
point at a loss of passivity.
The exposed surfaces, as in the exposure of different
types of specimens to the concrete environment, were
tested in the course of the electrochemical testing.
Namely, the cross-section of the wire, the outer surface
of the rod and the sheet were exposed to the simulated
pore water with a high alkalinity and to the pore water
with pH 9.2, containing 0.58 % of NaCl in order to simu-
late a carbonated and chloride-contaminated environment.
The corrosion potential was measured until it reached
a steady state, followed by a potentiodynamic linear
polarization and a wide potential scan at a higher scan
rate.
The corrosion potential after a exposure 2 h to the
pore water with a high pH shows minimum differences
among different samples. The results are presented in
Table 1. However, when exposed to the pore water with
pH 9.2, containing chlorides, the corrosion potential
changes and moves towards more negative values by
approximately 300 mV.
The linear-polarization technique showed that in the
simulated pore water, the surface of the steel sheet shows
the highest polarization resistance of 2014 kW cm2
(Table 1), followed by the carbon-steel rod (1260 kW cm2)
and the cross-section of the wire (246 kW cm2). With pH
9.2, the polarization resistance, Rp, for all the inve-
Figure 3: Open-circuit potential measurements for the three forms of
carbon steel: steel sheet, steel rod and steel wire, immersed in simulated pore water with pH 9.2, containing 0.58 % of chlorides
Slika 3: Meritve pri potencialu odprtega kroga za jekleno plo{~o, jekleno palico in prerez `ice, potopljene v simulirano raztopino betona,
pH 9,2 z 0,58 % kloridov
Table 1: Corrosion potential, polarization resistance and corrosion
rates for the carbon-steel sheet, carbon-steel wire and rod. Ecorr and Eb
are the values deduced from the potentiodynamic measurements.
Tabela 1: Korozijski potencial, polarizacijska upornost in korozijska
hitrost za jekleno plo{~o, `ico in palico. Vrednosti Ecorr in Eb so od~itane iz potenciodinamskih meritev.
pH = 12.8
wire sheet
Ecorr /V
–0.279 –0.280
Rp/(kW cm2) 246 2014
Eb/V
0.653 0.683
ncorr/(μm/year) 1.3
0.16
54
rod
–0.312
1260
0.648
0.24
pH = 9.2 + 0.58 %
NaCl
wire sheet
rod
–0.62 –0.56 –0.62
0.39 2.10 1.33
–0.422 –0.027 –0.099
773
143
227
Figure 4: Potentiodynamic measurements for the three forms of carbon steel: steel sheet, steel rod and steel wire, immersed in simulated
pore water, pH 12.5, and simulated pore water with pH 9.2, containing
0.58 % of chlorides at a scan rate of 0.1 mV/s
Slika 4: Meritve linearne upornosti za jekleno plo{~o, jekleno palico
in prerez `ice, potopljene v simulirano raztopino betona, pH 12,5 in
simulirano raztopino betona, pH 9,2 z 0,58 % kloridov pri hitrosti preleta 0,1 mV/s
Materiali in tehnologije / Materials and technology 48 (2014) 1, 51–57
A. ^ESEN et al.: CORROSION PROPERTIES OF DIFFERENT FORMS OF CARBON STEEL ...
stigated samples became smaller, since they underwent
corrosion processes. It is 595 times lower than at a high
alkalinity for the steel wire, 893 times lower for the steel
sheet and 840 times lower for the rod. Thereby, a lower
pH and a presence of chlorides greatly affect the corrosion resistance of the exposed steel surfaces. In an
aggressive environment, the aggressive species have no
preferential effect on the corrosion properties of different
investigated microstructures since the change in the
corrosion rate is very similar for all the samples.
In Figure 4, the potentiodynamic curves of the three
surfaces and three different microstructures (Figure 1)
are presented. There are small, but not negligible differences in the electrochemical properties in the simulated
pore water at pH 12.8 (Figure 4a).
The corrosion-current density is as high as jcorr =
0.123 μA/cm2 for the rod and jcorr = 0.053 μA/cm2 for the
cross-section of the wire. The carbon-sheet sample exhibits the lowest corrosion-current density (jcorr = 0.017
μA/cm2). The current densities in the pseudo passive
region in the anodic scans are the lowest for the steel
sheet, followed by the rod, and the highest currents are
found for the wire. The breakdown potentials are similar
at 0.65 mV vs. SCE (the results are presented in Table
1).
At a lower pH, the potentiodynamic curves are different for different samples. The corrosion potentials move
towards negative values, the corrosion-current densities
decrease and the breakdown potentials change. The
corrosion-current density is the highest for the carbon
wire (jcorr = 19.1 μA/cm2), smaller for the rod (jcorr = 7.72
μA/cm2) and the smallest for the carbon sheet (jcorr = 3.78
μA/cm2). Also, the breakdown potentials become smaller
as observed from the potentiodynamic curves and the
values given in Table 1.
Following that, the corrosion rates were estimated.
Any possible instances of crevice corrosion were elimi-
nated, so the corrosion rates were deduced from the
linear polarization using an equation, where the corrosion rate, ncorr, in μm/year is calculated following the
Faraday law18:
ncorr = 3.27 · (jcorr · w/M) / (d · n)–1
(9)
where jcorr stands for the corrosion-current density in
μA cm–2, d is the density of the metal in g cm–3, w is the
atomic mass (without units) and n is the number of
exchanged electrons. The steel density is d = 7.8 g cm–3
and the equivalent mass is w/M = 58.
jcorr was calculated from the equation, using the
Stern-Geary approximation of the Tafel coefficients:
jcorr = 1/2.303 · Rp (bA· bC/(bA + bC))
(10)
with bA and bC being 120 mV per decade.
As observed from these measurements, the corrosion
rate is low at a high alkalinity and it increases immensely
at a lower pH, especially when chlorides are introduced.
The effect of the microstructure is reflected in the corrosion performance. The carbon-steel sheet sample has
better corrosion properties than the carbon-steel rod,
whereas the carbon-steel wire is the most sensitive to
corrosion processes.
3.3 Surface characterization using SEM/EDX and the
Raman analysis
Different morphologies of corrosion products were
found on the steel surface: compact doughnut-type corrosion products (Figure 5a), fine rounded particles as
presented in Figure 5 b, a compact structure with visible
cracks in the surface (Figure 5c) and rounded particles
with a greater diameter (Figure 5 d).
All the identified corrosion products are iron-based
oxides or oxyhydroxides, some of them containing traces
of chlorine (Table 2).
Table 2: Mass fractions (w/%) of different elements in the corrosion
products on the steel sheet
Tabela 2: Masni dele` (w/%) razli~nih elementov v korozijskih produktih, najdenih na jekleni plo{~i
product
a
b
c
d
Figure 5: SEM images of the corrosion products on the carbon-steel
sheet after a 12-week exposure to wet and dry cycles in the mortar
Slika 5: SEM-prikaz korozijskih produktov po 12-tedenski izpostavitvi cikliranem mo~enju in su{enju v karbonatizirani malti
Materiali in tehnologije / Materials and technology 48 (2014) 1, 51–57
Fe
25.15
21.42
31.03
33.17
O
72.13
71.78
61.56
58.89
Na
6.58
4.25
Cl
2.29
0.22
0.44
2.59
Al
0.43
Si
0.76
0.79
0.89
0.30
The Raman analysis was also conducted on the steel
corrosion products. Since it is very difficult to link the
morphologies, found on SEM, with the optical magnifications, it was not possible to unambiguously recognize
each shape. The Raman spectra are presented in Figure 6.
Different types of iron oxides were identified. These
are hematite, lepidocrocite, gheothite, maghemite and
akaganeite. Different Raman spectra of the corrosion
55
A. ^ESEN et al.: CORROSION PROPERTIES OF DIFFERENT FORMS OF CARBON STEEL ...
bands characteristic for akaganeite are denoted. These
are (301, 380, 500, 550 and 710) cm–1, whereas the most
pronounced gheotite bands appear at (285, 399 and 534)
cm–1.
The corrosion products on the steel-carbon sheet
embedded in the mortar are very versatile. They were
investigated with SEM and their mineralogical nature
was identified with the Raman analysis. They consist of
different iron oxides and oxyhydroxides.
4 CONCLUSIONS
Figure 6: Raman spectra of diffrent corrosion products on the steelsheet surface
Slika 6: Ramanski spekter razli~nih korozijskih produktov na jekleni
plo~evini
products found on the steel-sheet surface are presented in
Figure 6.
Geothite is characterized by two strong bands at 306
cm–1 and 385 cm–1. The latter is broadened as a shoulder
of the peak at 410 cm–1. There is a weak band at 535
cm–1. Geothite is an a-FeOOH, iron oxyhydroxide, transparent and with an orthorombic structure. The spectra
were found in the literature.19 Hematite is defined by two
strong bands at 219 cm–1 and 283 cm–1 and three weak
and broad bands at (390, 490, 607 and 685) cm–1. The
results are similar to those reported in19,20.
Lepidocrocite, a g-FeOOH polymorph of iron oxyhydroxide, usually detected as a ruby-red corrosion
product, was detected at several points with the characteristic bands at 245 cm–1, 303 cm–1, 379 cm–1, 501 cm–1
and 680 cm–1. Similar spectra were already reported.19
Maghemite, g-Fe2O3, can be identified by broad and
non-intensive bands at 336 cm–1, 492 cm–1 and 668 cm–1.
This corrosion product is the most prevalent on the
examined surface.
At several spots, mixtures of different corrosion products were found, for example, a mixture of geothite and
akaganeite. In the Raman spectra in Figure 6, only the
56
Corrosion properties of three different forms of
carbon steel were investigated using metallography,
electrochemical and surface-spectroscopic techniques.
It was found that the microstructures of the three
investigated samples are different. This has an important
effect on the electrochemical properties of different samples of carbon steel.
At a high alkalinity of the simulated concrete-porewater solution with pH 12.8, the corrosion rates are low
and similar, while the effect of the microstructure is still
observed. The lowest corrosion rate was found for the
carbon-steel sheet, followed by the carbon-steel rod and
the wire, respectively. The microstructure of the wire is
the most sensitive one.
In the carbonated environment, in the concrete pore
water with pH 9.2, containing chlorides, the electrochemical properties of the investigated samples change
immensely. The corrosion proceeds and the corrosion
rate increases, causing a more expressed sensitivity of
the microstructure. The highest corrosion rate was found
for the wire that has the most sensitive microstructure,
followed by the carbon-steel rod and the sheet, respectively.
Different and versatile corrosion products were found
on the steel-carbon sheet embedded in the cement mortar
after an exposure to wet and dry cycles. They were investigated with SEM and their mineralogical nature was
identified with the Raman analysis. Different iron oxides
and oxyhydroxides were identified, such as geothite,
lepidocrocite, hematite, maghemite and akaganeite.
Acknowledgment
The help of Petra Mo~nik with laboratory experiments is greatly acknowledged as well as the help of
Viljem Kuhar with the metallographic analysis and helpful discussions.
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